1. Field of the Invention
The present invention relates to antenna structures, sensors using such structures, to a sensor array using such sensors, e.g. for detecting the presence of a target analyte, and in particular to sensors and sensor arrays where a target analyte is detected based on a detection of the photoluminescence of labels coupled to surface plasmon resonances (SPR). Moreover, the invention relates to an optical detection system comprising such antennas, sensors or sensor arrays, and to a method for detecting the presence of a target analyte using the sensor or sensor array.
2. Description of the Related Technology
Detecting low concentrations of a biological component (a “target”) in a fluid such as a liquid analyte mixture is a challenge. Often this is accomplished by selectively capturing the target with a binding composition, which is attached to a solid surface. Whether or not binding has occurred is detected by measuring the presence of a label that has either previously been attached to the target, or is linked to a second binding composition that binds to the captured target. Often the label is fluorescent such that detection is achieved by optical means in the linear regime. Two main elements determine the minimum target concentration that can still be detected. The first important factor is the detection sensitivity of the label. The more sensitive the detection the lower the number of bound labels that is needed before a positive identification can take place. The second factor is the presence of background signals, in particular signals from unbound labels. In the current state of the art these problems are often solved by first letting the binding reaction take place, and then washing away all the unbound labels before starting the detection of the bound labels. This is however undesirable since it increases the complexity of the assay and the measurement time. Different methods have been proposed to allow specific detection of the bound analyte for instance via the use of confocal excitation to reduce the excitation volume to an area of ˜1 μm above the surface. When very high background signals are expected this method is not sufficient.
Multi-photon absorption is a non-linear optical absorption process in which two or more photons, either of the same frequency or of different frequencies, are absorbed by a material to reach an excited state. The radiative relaxation from this excited state produces a photon with a higher frequency (shorter wavelength) than the exciting photons. Since the excitation occurs via a virtual state, linear absorption (one photon absorption) and emission of photons with emission at a lower frequency is forbidden. Two-photon absorption is a third-order nonlinear optical process, described by the third order non-linear optical susceptibility. The very low values of optical susceptibility of most materials means that two-photon absorption was of little interest. With the availability of suitable light sources (e.g. short-pulse lasers) and materials with large two-photon absorption cross sections (e.g. semiconductor quantum dots), this process is now receiving attention for technological applications in different fields such as solar energy generation and biophotonics. In a two-photon luminescence (TPL) measurement, the luminescence intensity scales quadratically with the excitation intensity. Significant two photon signals are obtained when the intensity is high, which has lead to the development of the powerful imaging technique of two photon luminescence microscopy for biological applications.
The intensity of luminescence I2-pho in a two photon excitation process is given byI2-pho=const·Pex2Δt where Pex is the power of the pump and Δt is the time of the exposure to the pump light.
In the case of a pulsed laser source I2-pho is given byI2-pho=const·Pex2Δt/(τ·f),where τ is the pulse duration and f is the laser repetition rate. For a mode-locked Ti:Sapphire laser providing a pulse train with a repetition rate of 80 MHz and a pulse duration of 200 fs, the two-photon fluorescence intensity is 62500 times larger than when a continuous wave (cw) laser with the same power is used. This difference can be compensated by increasing the power of the pump cw laser by a factor of 250. In this way, two photon absorption imaging of biomolecules has been demonstrated using cw lasers instead of with ultra short pulse lasers.
To make two-photon imaging and bio-sensing feasible at continuous wave excitation and low laser powers (<10 mwatt) is has been necessary to develop nanostructures which can locally increase the pump intensity due to resonant processes. Fluorescent probes in the proximity of these structures will exhibit modified properties. The optimization of these properties for biotechnology, the so-called radiative decay engineering, is nowadays in the centre of the research interest. Conductive nanostructures such as nanoantennas have been demonstrated to be very efficient local field enhancers due to the resonant excitation of particle plasmons polaritons or collective oscillations of free charge carriers. In the following we will refer to particle plasmon polariton resonanced as surface plasmon resonances (SPRs). Field enhancements of several orders of magnitude close to the metal surface have been predicted and measured in various structures. Enhancement of the two photon fluorescence of dyes in the proximity of silver nanoparticle fractal clusters has been demonstrated. Two photon luminescence using the field enhancement in single metallic nanorods have been used recently for in vivo imaging of vessels. Two photon luminescence detection of labelled proteins near silver colloid surfaces has been also reported.